(1) Field of the Invention
This invention relates to an abrasive composition for polishing a disc substrate for a magnetic recording medium, and a process for polishing the disc substrate. More particularly, it relates to an abrasive composition used for polishing a disc substrate for a magnetic recording medium having minimized surface irregularities, above which a magnetic head is capable of floating at a low flying height; and to a process for polishing the disc substrate.
(2) Description of the Related Art
Magnetic recording discs, i.e., memory hard discs, are widely used as memory means giving quick access to information as external memories of computers and word processors. A typical example of the magnetic recording discs is made by a process wherein a substrate composed of an aluminum alloy disc blank with a nickel-phosphorus (NiP) surface coating, formed by electroless plating, is surface-polished, and then, a chromium underlayer, a cobalt alloy magnetic film and a carbon protective film are formed in this turn on the polished NiP surface by a sputter deposition.
If the substrate has irregularities on the polished surface which have a height larger than the flying height of a magnetic read/write head, when the magnetic head floating above the magnetic disc flies at a high speed, the head impinges upon the irregularities with the result of damage of the magnetic head. If the substrate for the magnetic recording disc has polish marks or irregularities, the marks or irregularities develop on the surfaces of the chromium underlayer, the cobalt alloy magnetic film and the carbon protective film in turn, and consequently a functional defect is caused on the magnetic recording disc. Thus it is necessary to polish the substrate surface with a high precision for giving a magnetic recording disc having a smooth surface.
A wide variety of abrasive compositions and polishing methods have heretofore been proposed for removing or minimizing irregularities on the surface of substrate for magnetic recording discs and providing a smooth substrate surface with neither polish marks nor irregularities. As examples of the proposed abrasive compositions or polishing methods, there can be mentioned (1) a two-stage polishing method using an abrasive composition comprising aluminum oxide having incorporated therewith (i) an oxidizing agent such as sodium hypochlorite, or colloidal aluminum, and (ii) cerium dioxide (Japanese Unexamined Patent Publication [hereinafter abbreviated to "JP-A"] 60-108489), (2) an abrasive composition comprising alumina having incorporated therein sulfamic acid or phosphoric acid (JP-A 61-291674), (3) an abrasive composition comprising alumina having incorporated therein aluminum nitrate (JP-A 62-25187), (4) an abrasive composition comprising alumina having incorporated therein boehmite (JP-A 1-188264), (5) an abrasive composition comprising alumina having incorporated therein a metal salt and boehmite (JP-A 1-205973), (6) an abrasive composition comprising alumina having incorporated therein a metal nitrite salt (JP-A 2-158682), (7) an abrasive composition comprising alumina having incorporated therein boehmite and an ammonium salt of an organic or inorganic acid (JP-A 2-158683), (8) a polishing method using an abrasive composition comprising an alumina slurry having pre-treated with an ultrasonic filter (JP-A 3-106984), (9) an abrasive composition comprising alumina having incorporated therein boehmite and a water-soluble peroxide (JP-A 3-115383), (10) an abrasive composition comprising alumina having incorporated therein an amino acid (JP-A 4-108887), (11) an abrasive composition comprising alumina having incorporated therein (i) aluminum sulfate and/or aluminum chloride, and (ii) at least one compound selected from a peroxide, nitric acid, a nitrate salt, a nitrite salt and an aromatic nitro compound (JP-A 4-275387), (12) an abrasive composition comprising alumina having incorporated therein a chelate compound, boehmite and an aluminum salt (JP-A 4-363385), (13) an abrasive composition comprising alumina having incorporated therein boehmite which has been obtained by heat-treating rectangular primary particles of gibbsite (JP-A 5-271647), and (14) an abrasive composition comprising colloidal silica particles and a chemical etching agent (JP-A 7-240025).
In the above-mentioned proposals (1) through (13), finely divided alumina or other aluminum compounds having an average particle diameter of about 1 .mu.m are used as an abrasive, and thus, the substrate surfaces polished by these abrasive compositions exhibit a smoothness of an extent such that a practically acceptable flying height can be ensured. However, in recent years, a lower flying height is eagerly desired for allowing recording density to be increased, and it is difficult or impossible by these proposals to provide a perfect substrate surface with a high precision on which the desired lower flying height can be achieved.
In the above-mentioned proposal (14), colloidal silica particles having an average particle diameter of several-tens nanometer are used as an adhesive, a polished substrate surface with a high precision can be obtained. However, the rate of polishing is low and thus the productivity is low, and, when polishing is conducted for a long time, the circumferential edge part of a substrate surface is undesirably abraded in excess.
It is eagerly desired for an abrasive composition used for polishing an aluminum magnetic disc substrate for a high-density recording medium to produce a disc surface with a high precision such that a magnetic head is capable of floating at a low flying height. As regards the precision of disc surface, the magnitude of average surface roughness (Ra) and the presence of surface irregularities are important, and it is more important whether errors occur or do not occur when a read/write test is conducted as hereinafter described. It has recently been recognized that these errors are due to polish marks or polish pits. Therefore, it is eagerly desired to minimize or eliminate the occurrence of polish marks and polish pits.